Using computer modeling, we investigated the effect of interface roughness on the optical properties of a single junction a-Si:H solar cell by varying the root mean square roughness of the rough interfaces. The effect of the contact roughness on the external quantum efficiency, QE, of the solar cell in different parts of the light spectrum was determined. The choice of the back contact strongly influences the QE and the absorption in the non-active layers for the wavelengths above 650 nm. The effect of increasing the roughness of the contact interfaces was investigated for both the superstrate and substrate type cell. (C) 2001 Elsevier Science B.V. All rights reserved.